A seismic survey represents an attempt to image or map the subsurface of the earth by sending energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth, is reflected, and, upon its return, is recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3-D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data “cube” or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area. In reality, though, both 2-D and 3-D surveys interrogate some volume of earth lying beneath the area covered by the survey.
A conventional seismic survey is composed of a very large number of individual seismic recordings or traces. In a typical 2-D survey, there will usually be several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces. Chapter 1, pages 9-89, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, contains general information relating to conventional 2-D processing and that disclosure is incorporated herein by reference. General background information pertaining to 3-D data acquisition and processing may be found in Chapter 6, pages 384-427, of Yilmaz, the disclosure of which is also incorporated herein by reference.
A seismic trace is a digital recording of the acoustic energy reflecting from inhomogeneities or discontinuities in the subsurface, a partial reflection occurring each time there is a change in the elastic properties of the subsurface materials. The digital samples are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Each discrete sample in a conventional digital seismic trace is associated with a travel time, and in the case of reflected energy, a two-way travel time from the source to the reflector and back to the surface again, assuming, of course, that the source and receiver are both located on the surface.
Many variations of the conventional source-receiver arrangement are used in practice, e.g. VSP (vertical seismic profile) surveys, ocean bottom surveys, etc. Further, the surface location of every trace in a seismic survey is carefully tracked and is generally made a part of the trace itself (as part of the trace header information). This allows the seismic information contained within the traces to be later correlated with specific surface and subsurface locations, thereby providing a means for posting and contouring seismic data—and attributes extracted therefrom—on a map (i.e., “mapping”).
Of particular interest for purposes of the instant application are seismic exploration techniques such as VSPs or similar technology. By way of general background, a VSP survey is an exploration technique in which a seismic signal is generated at or near the surface and subsequently sensed by one or more geophones (land seismic sensors) or hydrophones (marine seismic sensors) that are situated in the subsurface, e.g., within a cased or uncased well which may or may not have been drilled for that purpose. VSP surveys have traditionally been used to create 1-D reflectivity maps and measure the travel-times to each geophone and their derivative, the near-borehole velocity field. VSP (and checkshot, see below) travel times are commonly used to calibrate surface seismic data to well depth via the explicit time-to-depth relationship measured by the well data. The near-borehole velocity field can be used for purposes such as updating the surface seismic velocity field, used for imaging, or used for pore pressure estimation via a transform. VSPs are also commonly used for offset surveys used for subsurface imaging. VSP imaging surveys may be 3-D (e.g., wherein a 2-D grid of surface source positions is utilized) and 2-D VSP (or walkaway VSP) which utilize a 1-D line of surface source positions along with the VSP array in the borehole.
VSP seismic data are often used to support and clarify the subsurface interpretation obtained from other seismic data sources (e.g., conventional surface seismic, well logs, cores, etc.). Because the VSP receivers are situated in the subsurface they potentially yield unique information about the up going and down going seismic energy and, since they are located much nearer to the subsurface target(s) of interest (and, in more particular, are located below the surface weathering layer) than surface receivers, there is an expectation that the data collected thereby will be yield a more representative image of the subsurface.
Related in general concept to the VSP survey is a checkshot survey, which also utilizes a surface source and downhole receivers (e.g., seismic receivers that are positioned within a producing well, a well that is being drilled, a well that was created for purposes of seismic imaging, etc.). However, the checkshot survey is directed not so much toward imaging the subsurface, but rather toward development of time-depth pairs for depth tying of surface seismic images to well data. In addition, checkshots can be used to create a velocity profile of the rocks near the well. One difference between a VSP survey and a checkshot survey is that in a checkshot survey attention is typically directed only toward the first breaks (earliest arrivals) of the seismic energy from the source, whereas in a VSP survey it is the seismic energy that is sensed following the first break that is most useful for purposes of seismic imaging. Of course, those of ordinary skill in the art will understand that a VSP survey also yields a checkshot survey, but not vice versa. Finally, the various methods of collecting and processing VSP and checkshot data to make them useful in seismic exploration are well known to those of ordinary skill in the art and, as such, will not be covered herein.
For all of the subsurface information that might be acquired via a VSP or other seismic survey, there are still geologic configurations that are not well imaged by these and other seismic techniques. As a specific example, obtaining accurate images of a salt dome flank in the Gulf of Mexico or elsewhere is a well known challenge. However, drilling wells based on imperfect or incomplete information about the location of the edge of salt risks an enormous financial loss if the well is mispositioned.
Heretofore, as is well known in the geophysical prospecting and interpretation arts, there has been a need for a method of using seismic data to obtain images of the subsurface that does not suffer from the limitations of the prior art. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of geophysical prospecting that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.